Lithography processing is a required and essential technology when manufacturing conventional integrated circuits. Many lithography techniques exist, and all lithography techniques are used for the purpose of defining geometries, features, lines, or shapes onto an integrated circuit die or wafer. In general, a radiation-sensitive material, such as photoresist, is coated over a top surface of a die or wafer to selectively allow for formation of the geometries, features, lines, or shapes.
A known method of lithography is generally referred to as photolithography. During a photolithographic step, photoresist is first formed over a top surface of a semiconductor wafer. A spin-on technique is usually used followed by a brief photoresist baking process. A mask having opaque regions, which are usually formed of chrome, and clear regions, which are usually formed of quartz, is positioned over the photoresist-coated wafer. Light is shone on the mask via a visible light source or an ultra-violet (UV) light source, such as a mercury-arc lamp or a mercury-vapor lamp. The light passes through all clear regions of the mask and is blocked by the opaque regions of the mask. The light exposure forms exposed portions of the photoresist on the semiconductor wafer and unexposed portions of the photoresist on the semiconductor wafer, respectively due to the clear and opaque regions of the mask.
In almost all cases, the light is reduced and focused via an optical system which contains one or several lenses, filters, and or mirrors before being exposed to the wafer. The optical system reduces feature sizes, sharpens imaging and in general improves the quality of the photolithographic process. Many techniques of photolithography such as contact printing, proximity printing, and projection printing are widely used in the integrated circuit industry. During contact printing the mask contacts the wafer during light exposure. During proximity printing the mask is positioned a short distance over the wafer. Projection printing usually involves multiple lenses wherein at least one lens is between the mask and the wafer. Photolithography, as conventionally practiced, is widely used but considered to be unfeasible for patterning features having a dimension less than 0.4 to 0.3 micron.
Another form of lithography is known as electron beam lithography or E-beam lithography (EBL). EBL technology allows for formation of small features without using a mask by focusing a thin electron beam onto a photoresist covered wafer. EBL is not used often in the integrated circuit industry due to the fact that EBL is extremely slow compared to other forms of lithography, and requires equipment which is usually three to five times more expensive than other widely used lithographic systems.
X-ray lithography is another technique that is used to form features onto a photoresist-covered wafer. X-ray lithography uses X-rays which are on the order of 0.4 to 5 nanometers in wavelength to form high-resolution features (i.e. very small features). Depth of focus problems, which are common to photolithography processing, are also reduced. X-ray lithography requires new photoresists or resist materials and can be cumbersome and expensive. In addition, X-ray masks are difficult to properly and consistently manufacture. Some known phenomena, such as laterial magnification error and blurring, are difficult problems that must be addressed when using X-ray lithography.
Another method of lithography which may be used for integrated circuit production is phase shifting. Phase shifting lithography is very similar to photolithography with the exception of the formation of the mask. A mask used for phase shifting lithographic systems has selectively-positioned phase shifting material and selectively positioned opaque material formed over a quartz plate. The quartz plate passes light which is referenced to be of a phase angle of 0.degree., and the phase shifting material passes light which is 180.degree. out of phase with the 0.degree. light. The 0.degree. light has a positive light intensity that alters the molecular weight of the photoresist. Additionally, the 180.degree. light also has a positive light intensity and alters the molecular weight of the photoresist in a manner identical to the 0.degree. light. The regions between the 0.degree. light and the 180.degree. light destructively interferes to form "dark" regions of low light intensity. These dark regions, which lie along vertical planes of the boundaries between the phase shifting material and the quartz material, form unexposed regions of the photoresist. Therefore, depending upon the use of negative or positive photoresist, sub-micron or sub-photolithographic lines and spaces can be formed using phase shifting techniques.
Due to the fact that phase shifting features are formed around the edges of various phase shifting regions via destructive interference, the destructive interference generates cylindrical or annular unexposed regions only. Although annular masking features are useful, other geometries such as lines, contacts, and snaked features are required for integrated circuit formation. Conventional single exposure phase shifting techniques therefore have limited application and use.